Valves and valve systems have been designed in the past having a valve actuated by a solenoid, piezoelectric stack or magnetorestrictive rod to control the flow of fluid through the valve system. The valve system may be realized as a common rail fuel injector, electrohydraulic actuator system, electronically-controlled fuel injector, gasoline port injector, fluid metering valve, relief valve, ventilation system, inhalators, reducing valve, direct valve or direct-injection gasoline injector, or any valve for controlling the flow of a flowing fluid or gas.
Flow control of a flowing fluid through a valve is generally achieved by manipulating the size of the valve opening between a valve gating member and a corresponding valve seat. Closing and/or opening of the valve opening generally involves manipulating a plunger and/or driving a valve member relative to its corresponding valve seat and/or valve opening.
However, in solenoid-controlled valve systems, it is often difficult to accurately control movement and positioning of the valve member through the control signals applied to the solenoids. This is especially true when intermediate positioning of a solenoid-controlled valve for example between the closed state and the open state, and maintained in a fixed position is desired.
Solenoid-controlled valves, by their very nature, are susceptible to variability in their operation due to inductive delays, eddy currents, spring preloads, solenoid force characteristics and varying fluid flow forces, overheating and disintegration of the valve housing generally due to overheating of the solenoid. Such weak spots must be carefully considered and accounted for in any solenoid-controlled valve system design. Moreover, the response time of the solenoids limits the minimum possible dwell times between valve actuations and makes the valve system generally more susceptible to various sources of variability.
While solenoids provide large forces and have long strokes, solenoids do have certain drawbacks. For example, first, during actuation, current must be continuously supplied to the solenoid in order to maintain the solenoid in its energized position. Further, to overcome the inertia of the armature and provide faster response times, a solenoid is driven by a stepped current waveform. A very large current is initially provided to switch the solenoid on; and after the solenoid has changed state, the drive current is stepped down to a minimum value required to hold the solenoid in that state. Thus, a relatively complex and high power current driver is required.
In addition to requiring a relatively complex and high current power source, the requirement of continuous current flow to maintain the solenoid at its energized position leads to heating of the solenoid. The existence of such a heat source, as well as the ability to properly dissipate the heat, is often of concern depending on the environment in which the solenoid is used, for example in plastic environment.
Additionally, the force produced by a solenoid is dependent on the air gap between the armature and stator and is not easily controlled by the input signal. This makes the solenoid difficult to use as a proportional actuator. Large proportional solenoids are common, but they operate near or at the saturation point and are very inefficient. Small, relatively fast acting non-proportional solenoids may have response times defined by the armature displacement as fast as 350 microseconds. However, this response time can be a significant limitation in some applications that require high repetition valve actuation rates or closely spaced events. Further, it is known that there is a substantial delay between the start of the current signal and the start of the armature motion. This delay is due to the inductive delay, experienced with solenoids, the delay experienced between the voltage and magnetic flux required to exert force on the armature. In valve systems, such delays lead to variability in the fluid flow.
While prior art electroactive actuators such as piezoelectric stacks and magnetorestrictive rods eliminate the response time and proportionality shortcomings of the solenoid, particularly in that it offers the benefit of using little power during while maintaining a static valve position for extended periods of time, while capable of handling large forces, however the valve stroke capabilities are very small. The output of these electroactive actuators must then be mechanically or hydraulically amplified, limiting the response time and proportionality benefits that they offer.
Because of their small strain capabilities, these actuators also tend to be large. Additionally, these actuators are uni-directional, i.e., they move in only one direction in response to a control signal. Therefore, any valve or mass moved by the actuator requires a return biasing force, for example with a return spring, to be applied to return the valve or mass to its original position. Often, the spring comprises a significant amount of the force required to move the valve or mass and represents another source of variability. Also, the beneficial response time of the actuator will have no impact on the return of the valve or mass, as it depends completely on the return spring.
Current valves are further prone to overheating, saturation, and exhibit hysteresis, all leading to inaccuracies with respect to controlling the flow through the valve such that the valve does not react linearly with respect to a control signal. Such factors lead to unpredictability of the valve and lack of true flow control through the valve body.
Other drawbacks of current piezoelectric valves is their inability to generate high holding forces and therefore provide valves with only low holding force that are not suitable for applications requiring larger holding forces.